X-ray differential phase-contrast (DPC) imaging relies on the refraction of the X-rays passing through an object. Since for hard X-rays the refraction angles are in the μ-radian range, the basic technique used for DPC imaging is to angularly filter with μ-radian resolution the transmitted X-ray beam, thus converting the angular beam deviations from refraction into intensity changes on a conventional detector. The angular filtering is done using X-ray optics such as crystals or gratings.
A fundamental advantage of DPC imaging is that it is sensitive to density gradients in the measured object rather than to its bulk X-ray absorption. In medical imaging for instance, refraction has a contrast enhancing effect at tissue boundaries, which enables the detection of soft tissues which are otherwise invisible in conventional X-ray imaging. The ultra-small angle scattering occurring in micro-structured soft tissue such as cartilage, tendon, ligament or muscle has also a volume contrast enhancing effect. Another benefit of DPC for medical imaging is that it can improve contrast and resolution at similar or lower dose than in conventional X-ray imaging. This is possible because DPC uses X-rays that are not absorbed by the body and because the soft tissue refraction coefficients decrease with X-ray energy much slower than the absorption ones. In particular, by using for DPC a spectrum with mean energy in the 50-80 keV range approximately, the soft tissue dose is minimized while refraction strongly dominates over absorption.
X-ray phase-contrast is also of interest for imaging and non-destructive characterization in material sciences, in particular as concerns low-Z materials. The structure and defects of materials ranging from polymers, to fiber composites, to wood, and to engineered bio-materials can be probed on the micrometer scale using X-ray phase-contrast. Some of the techniques used for X-ray phase-contrast can also be applied with neutrons. Recently X-ray phase-contrast has gained attention in fusion energy research, where the capability of refraction based imaging to measure the density gradients in an object can be used for the diagnostic of high density plasmas in inertial confinement fusion (ICF) and other high energy density physics (HEDP) experiments.
Until recently, research on X-ray DPC imaging has been done mostly at synchrotrons, using crystal optics; the high intensity of the synchrotron compensates for the low efficiency (less than a hundredth of a %) of the crystal optics. Although there are efforts to develop table-top synchrotrons, or to use narrow Kα lines from conventional tubes, the crystal method has not yet entered the domain of practical applications. It is thus of interest to develop more efficient DPC methods and optics, that can work with conventional medical or industrial X-ray tubes.
A DPC method that can work with conventional X-ray sources is the Talbot-Lau shearing interferometry, in which micro-periodic optics such as gratings are used to angularly filter the refracted X-rays with μ-radian resolution. The Talbot interferometer includes first a ‘beam-splitter’ (typically a π-shift phase grating), which divides (or ‘shears’) through the Talbot effect the incoming beam into few μ-radian wide beamlets. The Talbot effect consists in a ‘replication’ of the grating pattern by the wave intensity, at periodic distances along the beam, called Talbot distances, dT=k/η2·g2/(2λ), with λ the X-ray wavelength, g the grating period, k=1, 2, . . . the order of the pattern, and η=1 for a π/2 phase shifting grating or for an absorption grating, and η=2 for a it phase grating. The beamsplitter thus creates at the ‘Talbot distance’ a micro-periodic fringe pattern, which changes shape (shifts) with respect to the unperturbed pattern when a refractive object is introduced in the beam. The differential phase-contrast imaging consists thus in measuring the changes in the fringe pattern induced by the object, with respect to the pattern without the object. To achieve μ-radian angular sensitivity at hard X-ray wavelengths, the period g must be in the μm range, resulting in a Talbot distance of a few tens of cm.
The fringe pattern can in principle be directly measured using a microscopic pixel detector. This is however quite inefficient. For most practical applications, the fringe pattern changes are converted into intensity changes on a macroscopic pixel detector by introducing an ‘analyzer’ absorption grating placed behind the beam-splitter and having the period of the Talbot pattern. Lastly, for such an interferometer to function with an extended spot X-ray tube, a ‘source’ absorption grating is placed in front of the source, thus dividing it into an array of quasi-coherent line sources.
The gratings are made by micro-lithography in thin Si wafers or photoresist. The absorption gratings are difficult to fabricate; they are typically made by filling with gold the gaps in regular transmission gratings. The ‘grating shearing method’ described above has demonstrated performance similar to the crystal method at energies below a few tens of keV.
This method is however less useful at energies above a few tens of keV. The reason is that it is difficult to fabricate micron-period absorption gratings with the thickness required to block higher energy X-rays. This is illustrated in FIG. 1A with a plot of the Au thickness needed for 95% absorption, as a function of the photon energy. As seen, several hundred μm depth gratings would be needed in the range of interest for clinical DPC imaging. Depending on the grating period, the present technological limit is however around 50-100 μm. This limits the contrast of the grating shearing method for high energy X-rays, as illustrated in FIG. 1B by the fringe contrast computed for an interferometer having 100 μm thick, 4 μm period Au analyzer grating (throughout this specification we used for X-ray phase-contrast and optics calculations the XWFP wave propagation code and the XOP optics package).
Accordingly, it is desirable to develop improved types of optics to enable efficient DPC imaging at X-ray energies above a few tens of keV.